Date of this Version

March 1986

Abstract

Bubonic plague was first found in Los Angeles County in 1908. The largest epidemic of pneumonic plague in the United States occurred in the county in 1924, and the last cases of plague associated with domestic rodents in the United States occurred here in 1925. Sporadic plague activity was recorded from 1925 to 1975. Since 1975, plague has been found annually and is now endemic in the San Gabriel Mountains and the interface, that area where suburban encroachment intermingles with wilderness areas along the southern edge of these mountains. Within these two areas, plague is amplified and is a risk to humans when it occurs in the California ground squirrel, Spermophilus beecheyi. This rodent has been implicated directly in two human cases, and is now peridomestic throughout most of the interface area. A domestic cat was implicated with another case; the role of domestic pets in plague ecology is discussed. Although large populations of Rattus rattus exist within the interface, they currently play no role in plague ecology due to the virtual absence of fleas. The oriental rat flea, however, is seasonally very abundant in Rattus norvegicus living adjacent to the interface area and poses an alarming potential for epidemics if plague ever was introduced into this host population. The plague surveillance program in Los Angeles County centers on an active intelligence network to report signs of plague activity and on the combined use of serologies taken from wild carnivores and S. beecheyi. Early detection by these means plus active flea and ground squirrel suppression programs have been implemented to reduce plague activity and prevent human cases.